This invention relates generally to electrochemical cells in which a metal or a metal ion is the electroactive species, and more particularly to electrochemical cells including therein novel electrodes of the mixed-conducting matrix type.
There is a great deal of current interest in developing better methods for energy storage. Efficient energy storage is especially important for such applications as electric vehicles and the large scale storage of electrical energy during low demand periods for later use so that the load level of stationary power plants could be more evenly distributed and controlled. Electrochemical cells are particularly attractive for such applications. There has, therefore, been a concerted effort in recent times to develop energy and cost efficient electrochemical cells with improved performance characteristics.
The desirable adaptability of electrochemical cells for various applications stems from the fact that most of the energy associated with spontaneous chemical reactions taking place within these cells, can be converted into utilizable electrical energy by separating the reactive components--the positive and negative electrodes--such that the transfer of electronic charge takes place through an external circuit. The electrolyte allows and maintains the flow of ionic conduction between the two electrodes completes the electric circuit. Of the two classes of cells known, (primary and secondary cells), secondary cells are of particular interest as the associated chemical reactions are reversible by the application of electrical energy. The electrodes of secondary cells for practical applications must, therefore, be able to be regenerated several times during their life time.
Both kinetic and thermodynamic considerations place severe limitations on the performance of all practical electrochemical cells or battery systems. The voltage across any cell is directly related to thermodynamic parameters of the reaction. In order to be stable and reversible in the operational range, electrode potentials have to be within a specified range which is restricted by the properties of the electrolyte of choice. At the same time, the overall electrochemical reaction must occur at a reasonably adequate rate in order to be practical. The development of advanced cells, therefore, depends on the qualities and choice of appropriate materials for the electrodes and for the electrolyte. In addition, the geometric configuration of the electrodes for optimum or desired performance must be established.
One of the current areas of research involves electrochemical cells in which alkali metals or their ions serve as the electroactive species. Major advantages of such batteries include more efficient energy storage, better energy or power to weight or volume ratios and potentially higher voltages compared to conventional batteries. Lithium is a particularly attractive candidate since it is a very electropositive, reasonably abundant, light weight element and is relatively easy to handle. In cells which utilize lithium as the electroactive species, the maximum voltage is obtained if the negative electrode is elemental lithium, but practical considerations such as rechargeability, safety and other factors limit its use in the elemental form. Elemental lithium is unstable in oxidizing environments and liquefied lithium can react violently with the electrolyte or other components normally found in such battery systems. As a result, alloys of lithium are sometimes used instead of elemental lithium in negative electrodes. In one exemplary situation where the negative electrode is an alloy of lithium with aluminum or lithium with silicon, the positive electrode may be a metal sulfide such as iron sulfide and the electrolyte may be a molten salt, such as for example, the eutectic composition of the lithium chloride-potassium chloride system. Due to the high melting point of these salts, such cells are normally operated in the temperature range of about 350.degree. C. to about 500.degree. C.
The use of elemental lithium electrodes in such high temperature lithium/metal sulfide battery systems would present additional, serious problems. It is highly corrosive, difficult to contain, and dissolves in the molten salt electrolyte, causing severe self discharge. These problems can be to a large extent alleviated by using metallic lithium and/or solid lithium-metal alloys; but electrodes made out of such alloys exhibit much slower kinetics than elemental lithium. In order to compensate for this lower kinetics and to provide the requisite current densities, a large surface area for the electrodes becomes a necessity. Such lithium based electrodes which provide a large surface area, are typically made either as a highly porous salt-filled sponge, such as the lithium-aluminum (Li-Al) system, or as a powder contained in a mesh cage, such as the lithium-silicon (Li-Si) system. These lithium-metal alloys are often found to lose their charge-holding capacity with frequent cycling. With the "porous" electrodes of the Li-Al type, their porosity, mechanical and shape instability and manufacturing difficulties pose additional problems. Furthermore, porous electrodes (porous structures permeated by the electrolyte), present the possibility of electrolyte freezing due to high, local charge flux densities and consequent local compositional changes.